1. Field of the Invention
The present invention relates to carrier frequency conversion and more particularly to the use of non-linear materials to change a carrier frequency while maintaining the overall rate of information transfer constant.
2. Description of the Related Art
In communications, there is a need to readily and rapidly change the carrier frequency as the information traverses different transmission media. In the specific realm of communications in which information is carried through the modulation of electromagnetic radiation, there is often a need to change from one carrier frequency to another. The reason for this change may be associated with different hardware processing or transportation platforms. It may be due to a transition between the optical frequency carriers associated with fiber optical communications and lower frequency (microwave) carriers used for free space information transmission. Since these communications systems' technical evolution occurred independently, techniques for information transfer between systems have been developed on an ad hoc basis. Typically, this means that the electromagnetic radiation is detected and changed into an electronic format, which is then sent to a different electromagnetic radiation source at the new carrier frequency and transmitted at that new frequency. Specifically, to change carrier frequency from optical to radio frequency (rf), an optical photodetector with rf bandwidth is used to demodulate carrier down to baseband and signal is re-transmitted as rf. In order to convert from rf to optical, the rf carrier is demodulated to baseband by rf mixer and the optical carrier generated by re-transmitting the signal using a diode laser. Clearly, this additional step of changing the radiation to an electronic format and rebroadcasting at a new frequency causes considerable information bottlenecking.
A preferred method would be to convert between the carrier frequencies by direct electromagnetic mixing. In the case of rf carrier frequencies, carrier frequencies are up- and down-converted using rf frequency mixers. Nonlinear optical processes such as sum-frequency mixing or difference frequency mixing work well when both carriers are in the optical frequency regime.
Furthermore, in the case where the radiation is being used to transport digital information, the information transfer rate is fundamentally limited by the carrier frequency. For example, a carrier in the optical frequency range can transport approximately 103 times more information per second than a carrier at microwave frequencies, due to the former's higher carrier frequency. The so-called information bandwidth is given by, IB=IC×N, where IC is the frequency of the carrier and N is the number of carriers operating in that frequency regime. Unless N changes, IB drops precipitously when the carrier frequency drops. This loss of carrier rate can be addressed by using a proportionally larger number of carriers (N) at the lower frequency. However, this entails additional processing of the information while it is in the electronic mode between the two carriers as previously described, causing the bottleneck to worsen. It is desirable to be able to change carrier frequency to accommodate a change in the data transfer capability of the transmission medium and is further desirable to accomplish this without changing the electromagnetic radiation to an electronic signal first.
Non-linear optical (NLO) materials are unusual in that they allow electromagnetic radiations or light to directly interact with other light, with the material acting only as the mediating medium. One such example is disclosed in U.S. Pat. No. 6,441,949, issued to T. A. Reynolds et. al., which discusses the use of polyborates, materials formed from divalent metal ions and borate anions. Art in that patent discusses optical parametric applications, but does not mention applications to radiation production at microwave or terahertz frequency production.
Several patents exist which discuss means to generate terahertz electromagnetic radiation. U.S. Pat. No. 6,144,679 discusses a means to generate a coherent terahertz source and U.S. Pat. No. 5,543,960 discusses a particular mosaic of electro-optical crystals to perform this task. Neither recognizes discusses the production of a broadband terahertz wave.
Furthermore, related art also exists in the public literature. Papers by Xu et. al. Applied Physics Letters 61 (15), p 1784, 1992 and Zhang et. al. Applied Physics Letters 61 (15) p. 2764, 1992 discuss the physics associated with optical rectification. There are no applications discussed in either of these papers and no mention of use within the sphere of optical communications.